The present invention relates to a data transfer control method for controlling the data transfer between a rotary memory device such as, for example, a disk drive apparatus and a central processing unit by commands from the central processing unit.
Hitherto, there has been known a disk control apparatus for transferring the read data of a disk drive apparatus for a central processing unit, which disk control apparatus controls the disk drive apparatus such that positioning operation to a desired record is started after receiving the command (LOCATE command) to instruct the read start position and the total number of bytes to be transferred by the read command chained to the LOCATE command, but the loading of data into the data buffer is started only when the read command which is next to the LOCATE command in command chain is received. However, in this method, if the interruption signal is not accepted by the channel, the disk control apparatus must then wait for the next rotation, thereby resulting in low efficiency. In particular, if the state in which the interruption signal cannot be accepted continues for a time longer than a predetermined period of time, an error such as time-over or the like will occur. An attempt for overcoming this problem is disclosed in, for example, JP-A-55-34783. However, according to this method, there is the prerequisite such that only after the positioning completion interruption has not accepted more than a predetermined number of times, the completion interruption is always signaled. Therefore, there exists a state in which the completion interruption is not received a few times.
In the case where the data transfer speed on the side of the central processing unit is faster than that of the external memory device, if the loading of the data into the buffer is started after the reception of the read command which is next to the LOCATE command in the command chain, the data transfer on the side of the CPU and the data transfer on the side of the external memory device will simultaneously start. However, in this case, since the data transfer speed on the side of the CPU is faster, the CPU must wait until the desired record is fully stored into the buffer or must execute the data transfer in accordance with the data transfer speed on the side of the external memory device. Thus, the connection time with the CPU is prolonged beyond what it is needed.